Optical scanning device

ABSTRACT

An optical scanning device ( 1 ) for scanning an information layer ( 2 ) includes a radiation source ( 6 ) for supplying a radiation beam ( 14 ), a lens system ( 7 ) having an optical axis (OO′), and a detection system  8 ) including: (i) an astigmatism generating element ( 9 ) for generating a first amount of astigmatism (W 1 ) represented by a vector (W o,1 , θ 1 ), so as to transform the radiation beam to a first astigmatic radiation beam ( 29 ); (ii) an astigmatism correcting element ( 27 ) for generating a second amount of astigmatism (W 2 ) represented by a vector (W o,2 , θ 2 ), so as to transform the first astigmatic radiation beam to a second astigmatic radiation beam ( 30 ) having a third amount of astigmatism (W 3 ) represented by a vector (W o,3 , θ 3 ), and (iii) a detector ( 28 ) for transforming the second astigmatic radiation beam to an electrical signal. According to the invention, W 3  is adapted to the detector and that W o,2  and θ 2  comply substantially with the following equation: (W o,1 , 2θ 1 )+(W o,2 , 2θ 2 )=(W o,3 , 2θ 3 ).

The invention relates to an optical scanning device for scanning aninformation layer, the device including (a) a radiation source forsupplying a first radiation beam, (b) a lens system for transformingsaid first radiation beam to a scanning spot in said information layer,the lens system having an optical axis, and a (c) detection systemincluding:

-   -   an astigmatism generating element for generating a first amount        of astigmatism W₁ so as to transform said first radiation beam        to a first astigmatic radiation beam having a first focal line        and a second focal line which is further from said astigmatism        generating element than said first focal line, said first amount        of astigmatism being represented by a vector (W_(o,1), θ₁) in a        reference plane perpendicular to said optical axis, where        W_(o,1) represents the magnitude of W₁ and θ₁ represents the        angle between said first focal line and a reference axis which        is perpendicular to said optical axis;    -   an astigmatism correcting element for generating a second amount        of astigmatism W₂ so as to transform said first astigmatic        radiation beam to a second astigmatic radiation beam having a        third amount of astigmatism W₃, said second amount of        astigmatism being represented by a vector (W_(o,2), θ₂) in said        reference plane, and said third amount of astigmatism being        represented by a vector (W_(o,3), θ₃) in said reference plane,        and    -   a detector for transforming said third radiation beam to an        electrical signal.

In the present description, “scanning an information layer” refers toscanning with a radiation beam for reading information from theinformation layer (“reading mode”), writing information on theinformation layer (“writing mode”), and/or erasing information from theinformation layer.

In the present description, an amount of astigmatism is characterized asfollows. Considering an optical element having an optical axis, anentrance surface and an exit surface and that the optical elementgenerates an amount of astigmatism W so as to transform an incident,non-astigmatic radiation beam to an emerging, astigmatic radiation beamhaving a first focal line and a second focal line which is further fromthe exit surface than the first focal line. The amount of astigmatism Wis represented by a vector (W_(o), θ) in a reference plane (XY)perpendicular to the optical axis, where W_(o) represents the magnitudeof W and θ is an angle that represents the direction of W. The magnitudeW_(o) is represented by a Seidel coefficient; alternatively, it may berepresented by a Zernike coefficient, a peak-value of the wavefrontaberration, the “astigmatic distance” or longitudinal aberration (whichcorresponds to the distance between the first and second focal lines).The angle θ is represented by the angle between the direction of thefirst focal line and a reference axis (X) which is perpendicular to theoptical axis.

Generally speaking, it is important to keep the scanning spot on track,that is, to maintain the scanning spot in focus on the information layerwhich is to be scanned and to maintain the scanning spot on the centerline of the track to be scanned. In the following, for a given track,the “radial direction” means the direction between the track and thecenter of the disc and the “tangential direction” means the directionthat is tangential to the track and perpendicular to the radialdirection.

In order to maintain the scanning spot in focus on the information layerwhich is to be scanned, a “focus error signal” is commonly formedaccording to the so-called “astigmatic method” which is known from,inter alia, the book by G. Bouwhuis, J. Braat, A. Huijser et al,“Principles of Optical Disc Systems,” p.75–80 (Adam Hilger 1985) (ISBN0-85274-785-3). This method is based upon an optical aberration calledastigmatism which is deliberately introduced in the optical path of theradiation beam. More specifically, the astigmatism generating elementtransforms the radiation beam to a radiation beam having an astigmatismthat has an angle (preferably 45°) with respect to the radial direction.The detector transforms this astigmatic radiation beam to the focuserror signal, this signal being further used for mechanically adjustingthe position of the lens system along its optic axis for maintaining thescanning spot in focus on the information layer. When the scanning spotis in focus on the information layer, the shape of the spot on thedetector is circular and therefore the focus error signal equals 0. Whenthe scanning spot is too close to or too far from the information layer,the radiation beam reflected by the disc via the lens system isdivergent or convergent: the shape of the spot on the detector iselliptical and therefore the focus error signal differs from zero.

In order to maintain the scanning spot in the right track, a“radial-tracking error signal” may be formed according to the so-called“radial push-pull method” known from, inter alia, said book by G.Bouwhuis et al, p. 70–73. This signal is further used for mechanicallyadjusting the position of the lens system along the radial direction formaintaining the scanning spot in the track of the information layerwhich has to be scanned.

FIG. 1 a of the present description shows a known optical scanningdevice in a known configuration which enables the generation of thefocus error signal and the radial tracking error signal. The knowndevice scans a track of an optical record carrier along the center lineof the track. With reference to FIG. 1 a, “X_(o)” and “Y_(o)” are tworeference axes parallel to the radial direction and the tangentialdirection, respectively, and “Z_(o)” is a reference axis which forms,together with the axes X_(o) and Y_(o), an orthogonal base. The knowndevice includes two shafts Sa and Sb and an optical scanning head OH.The optical scanning head is able to move along the shafts Sa and Sb, ina direction parallel to the axis X_(o), thereby allowing scanning fromone track to another track and adjusting the position of the scanningspot on the center line of the track to be scanned. The optical scanninghead OH includes a radiation source RS, a lens system LS and a detectionsystem DS. The lens system LS includes a collimator lens CL, anobjective lens OL and a plane mirror (not shown in FIG. 1 a). The planemirror is arranged below the objective lens OL along the axis Z_(o). Thenormal to the plane of the mirror divides the angle of 90° between theaxis Z_(o) and a reference axis Z. As shown in FIG. 1 a, “Z” is thereference axis having the same direction as the optical axis between thedetector D and the collimator lens CL. The axis Z makes an angle of 45°with the tangential direction Y_(o). During scanning, a radiation beamis emitted from the radiation source RS and directed to the track to bescanned via the lens system LS. The radiation beam then reflects on thetrack to be scanned in the form of a main reflected radiation beam anddiffraction radiation beams (mainly a +1^(st)-order diffraction beam anda −1^(st)-order diffraction beam). The diffraction beams have the shapeof half-lobes in the cross-section of the main radiation beam. The mainreflected radiation beam and the +1^(st)-and −1^(st)-order diffractionbeams are directed to the detection system DS via the lens system LS.The detection system DS includes (1) a plane parallel plate PPP used asboth a beam splitter and an astigmatism generating element and (2) thedetector D. In the configuration shown in FIG. 1 a, the orientation ofthe plate PPP causes the focal lines of beams as generated by theastigmatism to have an angle of 0° or 90° with a reference axis N whichis perpendicular to the axis Z and contained in the plane of thedrawing. The direction of the track (that is, the tangential directionalong the axis Y_(o)) and the orientation of the mirror cause the+1^(st) order diffraction lobe and the −1^(st) order diffraction lobe tohave angles of 45° with the axis N on the detector D. In this case, theangle of 45° between the focal lines and the lobes allows use of aquadrant detector as the detector D, to generate a radial error signaland a focus error signal using the push-pull method and the astigmaticmethod, respectively.

However, for mechanical reasons such as mechanical space, it isadvantageous to have an orientation of 0° or 90° as shown in FIG. 1 b.

The known device of FIG. 1 b has the same components than those of theknown device of FIG. 1 a but arranged in a different configuration. Withreference to FIG. 1 b, “X₁” and “Y₁” are two reference axes parallel tothe radial direction and the tangential direction, respectively, and“Z₁” is a reference axis which forms, together with the axes X₁ and Y₁,an orthogonal base. The known device of FIG. 1 b scans a track of anoptical record carrier along the center line of the track, which isparallel the axis Y₁. The optical scanning head OH is able to move alongthe shafts Sa and Sb, in a direction parallel to the axis X₁. The planemirror of the lens system LS (not shown in FIG. 1 b) is arranged belowthe objective lens OL along the axis Z₁. The normal to the plane of themirror divides the angle of 45° between the axes X₁ and Z₁.

However, in the configuration shown in FIG. 1 b, the optical scanningdevice has the disadvantage that, without further measures, it does notenable to form the focus error signal and the radial tracking errorsignal simultaneously. As explained with respect to FIG. 1 a, theorientation of the plate PPP causes the focal lines of beams asgenerated by the astigmatism to have an angle of 0° or 90° with the axisN. The direction of the track (that is, the tangential direction alongthe axis Y₁) and the orientation of the mirror cause the +1^(st) orderdiffraction lobe and the −1^(st) order diffraction lobe on the detectorD to have an angle of −90° or +90° with the axis N. In this case, byusing a quadrant detector as the detector D, the mutual orientation ofthe lobes and the focal lines does not allow to generate simultaneouslya radial error signal and a focus error signal by using the push-pullmethod and the astigmatic method, respectively.

A solution to remedy this disadvantage is known from U.S. Pat. No.4,731,527. According to the known solution, the detection system furthercomprises an astigmatism correcting element formed by a cylindricallens. As explained in U.S. Pat. No. 4,731,527, the astigmatismgenerating element generates a first amount of astigmatism W_(a) and theastigmatism correcting element generates a second amount of astigmatismW_(b). The cylindrical lens is arranged so that the amount ofastigmatism W_(c) is made to provide the maximum sensitivity of thequadrant detector, i.e. to be oriented at 45° with respect to the radialdirection according to the “astigmatic method.” This is achieved,according to U.S. Pat. No. 4,731,527, when the amounts of astigmatismW_(a), W_(b) and W_(c) meet the following equation:W _(c) =W _(a) +W _(b)  (1)

According to Equation (1), FIG. 2 of the present description shows therelationship between the amounts of astigmatism W_(a), W_(b) and W_(c)in a reference plane XY that is perpendicular to an optical axis of thedetection system where W_(o,a), W_(o,b) and W_(o,c) are the magnitudesof W_(a), W_(b) and W_(c), respectively, and θ_(a), θ_(b) and θ_(c)which are the angles of W_(a), W_(b) and W_(c), respectively. By usingthe vector representation of the amounts of astigmatism, Equation (1)may also be expressed as follows:(W _(o,c), θ_(c))=(W _(o,a), θ_(a))+(W _(o,b), θ_(b))  (2)

However, the teaching of U.S. Pat. No. 4,731,527 is unsatisfactory,since it does not provide the desired correction of the astigmatism.

Other optical scanning devices including an astigmatism correctingelement are disclosed in the prior art, such as U.S. Pat. No. 4,968,874.

U.S. Pat. No. 4,968,874 discloses an optical scanning device asdescribed in the opening paragraph wherein the astigmatic correctingelement includes a first anisotropically curved surface for compensatingthe astigmatism generated by the astigmatism generating element and asecond anisotropically curved surface for generating a new astigmatismin an arbitrary direction, that is, 45° with respect to the radialdirection.

A disadvantage of the astigmatism correcting element described in U.S.Pat. No. 4,968,874 is that it includes two anisotropically curvedsurfaces which are difficult to manufacture.

An object of the invention is to provide an optical scanning device thatremedies the aforementioned disadvantages, in particular, that providesastigmatism aberration according to the “astigmatic method” and that iseasy to manufacture.

In accordance with the invention, these objects are achieved by anoptical scanning device as described in the opening paragraph, which ischaracterized in that W₃ is adapted to said detector and that W_(o,2)and θ₂ comply substantially with:(W _(o,1), 2θ₁)+(W _(o,2), 2θ₂)=(W _(o,3), 2θ₃).

An advantage of the optical scanning device according to the inventionis that it allows the skilled person to correct the astigmatism asdesired, as shown below in further detail (in particular in relation toTables 3 and 4).

In a preferred embodiment of the optical scanning device, theastigmatism correcting element is formed by a cylindrical surface havingan axis of symmetry that forms a predetermined first angle with respectof the first focal line, the value of this angle depending on thedesired angle, the first astigmatic distance and the second astigmaticdistance.

An advantage of forming the astigmatism correcting element with acylindrical surface is that it is easier to manufacture oneanisotropically curved surface than two anisotropically curved surfaceslike those described with reference to U.S. Pat. No. 4,968,874. In thepresent description, an “anisotropically curved surface” means a surfacewith different curvatures and/or aspherical coefficients in two mutuallyperpendicular directions.

Another advantage of an astigmatism correcting element with acylindrical surface is that the cylindrical surface is less sensitive tomechanical tolerance changes than an anisotropically curved surface,like those of the astigmatism correcting element described in U.S. Pat.No. 4,968,874.

In a preferred embodiment of the optical scanning device, theastigmatism generating element is formed by a plane parallel plate thatis tilted with a predetermined angle between the normal direction ofthis plate and the optical axis. This embodiment can also be used asbeam splitter to redirect the radiation beam from the radiation sourcein the direction of the lens system.

An advantage of forming the astigmatism generating element with a planeparallel plate is that the plane parallel plate is cheaper than otherastigmatism generating element that can be found in the commerce, suchas a beam splitting cube.

Another advantage of forming the astigmatism generating element with aplane parallel plate is that the plane parallel plate is generallythinner than a beam splitting cube, thereby leading to a more compactoptical scanning device.

The objects, advantages and features of the invention will be apparentfrom the following, more detailed description of the invention, asillustrated in the accompanying drawings, in which:

FIGS. 1 a and 1 b show a known optical scanning device in a firstconfiguration and a second configuration, respectively;

FIG. 2 is a graph showing the relationship between three amounts ofastigmatism according to the teaching of a prior art document,

FIG. 3 is a schematic illustration of components of an optical scanningdevice according to one embodiment of the invention,

FIG. 4 is a graph showing the relationships between three amounts ofastigmatism, according to the present invention,

FIG. 5 is a schematic illustration of a first embodiment of thedetection system shown in FIG. 3

FIG. 6 is a schematic illustration of a second embodiment of thedetection system shown in FIG. 3

FIGS. 7 and 8 are schematic representations of the central radiationbeam formed on the quadrant detector shown in FIGS. 5 and 6, with andwithout correction according to the invention, respectively, and

FIGS. 9 and 10 are schematic representations of the satellite radiationbeams formed on the quadrant detector shown in FIGS. 5 and 6, with andwithout correction according to the invention, respectively.

FIG. 3 is a schematic illustration of components of an optical scanningdevice 1 according to the invention for scanning an information layer 2of an optical record carrier 3. The configuration of the componentsshown in FIG. 3 is the same than the configuration of FIG. 1 b; however,the mirror of the lens system described with reference to FIG. 1 b hasnot been drawn in FIG. 3 for clarity reasons. Notably, as a matter ofpurely arbitrary choice and with reference to FIG. 3 and seq., “X-axis”and “Y-axis” are reference axes corresponding to the radial directionand the tangential direction of the optical record carrier,respectively, and “Z-axis” is a reference axis parallel to an opticalaxis in the optical scanning device 1.

By way of illustration, the optical record carrier 3 includes atransparent layer 4 on one side of which the information layer 2 isarranged. The side of the information layer 2 facing away from thetransparent layer 4 is protected from environmental influences by aprotective layer 5. As shown in FIG. 3, the layers 2, 4 and 5 are planarin the directions of the X-axis and the Y-axis indicated in FIG. 3. Thetransparent layer 4 acts as a substrate for the carrier 3 by providingmechanical support for the information layer 2. Alternatively, thetransparent layer 4 may have the sole function of protecting theinformation layer 2, while the mechanical support is provided by a layeron the other side of the information layer 2, for instance by theprotection layer or by an additional information layer and transparentlayer connected to the uppermost information layer. The informationlayer 2 is a surface of the carrier 3 that contains tracks. A “track” isa path to be followed by a focused radiation beam on which pathoptically-readable marks representative of information are arranged. Themarks may be, e.g., in the form of pits or areas with a reflectioncoefficient or a direction of magnetization different from thesurroundings. In the case where the optical record carrier 3 has theshape of a disc, the following is defined with respect to a given track:the “radial direction” is the direction between the track and the centerof the disc and the “tangential direction” is the direction that istangential to the track and perpendicular to the “radial direction.”

As shown in FIG. 3, the optical scanning device 1 includes a radiationsource 6, a lens system 7 having an optical axis OO′, and a detectionsystem 8. The X-axis is perpendicular to the optical axis OO′ andparallel to the radial direction. The Y-axis is perpendicular to theoptical axis OO′ and to the X-axis. The Z-axis is parallel to theoptical axis OO′ and therefore is perpendicular to both the X-axis andthe Y-axis.

As also shown in FIG. 3, the optical scanning device 1 preferablyfurther includes a beam splitter, a servo circuit 10, a focus actuator11 and a radial actuator 12, and an information processing unit forerror correction 13.

The radiation source 6 is arranged for supplying a radiation beam 14.Preferably, the radiation source 6 includes at least one semiconductorlaser that emits the radiation beam 14 at a selected wavelength λ. Forinstance, in the case where the optical record carrier 3 is of theso-called DVD format, the wavelength λ of the radiation beam 14 isbetween 620 and 700 nm and, preferably, equals 660 nm and, in the casewhere the optical record carrier 3 is of the so-called DVR-format, thewavelength λ preferably equals 405 nm. More preferably, the radiationsource 6 includes a grating structure 6′ for forming two satelliteradiation beams (not shown in the figures) from the central radiationbeam 14; the satellite beams are used for generating the radial-trackingerror signal.

The beam splitter is arranged for reflecting the radiation beam 14 (aswell as the two satellite radiation beams) toward the lens system 7. Inthe preferred embodiment shown in FIG. 3, the beam splitter is formed bya plane parallel plate 9 that is tilted with respect to the optical axisOO′ so as to form an angle α with respect to this axis. Preferably, theangle α equals 45°. Alternatively, the beam splitter may be formed by agrating structure or a hologram.

The lens system 7 is arranged for transforming the radiation beam 14 toa focused radiation beam 17 so as to form a scanning spot 18 in theposition of the information layer 2. Preferably, the lens system 7includes a first objective lens 19. It further includes a collimatorlens 20 and a second objective lens 21. Preferably, the second objectivelens 21 is used together with the first objective lens 20 in the casewhere the numerical aperture of the radiation beam 17 approximatelyequals 0.85, while only the first objective lens 20 is used in the casewhere the numerical aperture of the radiation beam 17 is smaller than0.65.

The collimator lens 20 is arranged for transforming the radiation beam14 (as well as the two satellite radiation beams) into a substantiallycollimated radiation beam 22.

The objective lens 19 is arranged for transforming the collimatedradiation beam 22 to a converging radiation beam 23. The objective lens19 has an entrance surface 19 a for receiving the radiation beam 22 andan exit surface 19 b for outputting the converging beam 17.

The second objective lens 21 is arranged for transforming the convergingradiation beam 23 to the focused radiation beam 17. The lens 21 may be aplano-convex lens having a convex entrance surface 21 a that faces theexit surface 19 b of the objective lens 19, and a flat exit surface 21 bthat faces the position of the information layer 2. Notably, the secondobjective lens 21 forms, in cooperation with the first objective lens19, a doublet-lens system that has advantageously a larger tolerance inmutual position of the optical elements than a single-lens system.Furthermore, one or two surfaces of each of the objective lenses 19 and21 are preferably aspherical.

By way of illustration, in the case where the optical record carrier 3is of the DVD format, the numerical aperture of the focused radiationbeam 17 approximately equals 0.6 for the “reading mode” and preferably0.65 for the “writing mode.”

During scanning, the forward focused radiation beam 17 reflects on theinformation layer 2, thereby forming a backward radiation beam 24 whichreturns on the optical path of the forward focused radiation beam 17.The lens system 7 transforms the backward radiation beam 24 to a firstbackward radiation beam 25. Finally, the beam splitter 9 separates theforward radiation beam 14 from the backward radiation beam 25 bytransmitting at least part of the backward radiation beam 25 towards thedetection system 8.

The detection system 8 is arranged for capturing the radiation beam 25(as well as the corresponding satellite radiation beams not shown inFIG. 3) and converting them into one or more electrical signals. One ofthe signals is an information signal I_(data), the value of whichrepresents the information scanned on the information layer 2. Theinformation signal I_(data) is processed by the information layerprocessing unit 13 for error correction. Other signals from thedetection system 8 are a focus error signal I_(focus) and a radialtracking error signal I_(radial). The signal I_(focus) represents theaxial difference in height along the optical axis OO′ between thescanning spot 18 and the position of the information layer 2.Preferably, this signal is formed by the “astigmatic method” asdescribed above. The signal I_(radial) represents the distance in theplane of the information layer 2 between the scanning spot 18 and thecenter line of a track on the information layer 2 to be followed by thescanning spot 18. Preferably, the signal I_(radial) is formed from the“radial push pull method” as described above.

The servo circuit 10 is arranged for, in response to the signalsI_(focus) and I_(radial), providing servo control signals I_(control)for controlling the focus actuator 11 and the radial actuator 13,respectively. The focus actuator 11 controls the positions of theobjective lenses 19 and 21 along the optical axis OO′, therebycontrolling the actual position of the scanning spot 18 such that itcoincides substantially with the plane of the information layer 2. Theradial actuator 13 controls the position of the objective lenses 19 and21 in a direction perpendicular to the optical axis OO′, therebycontrolling the radial positions of the scanning spot 18 such that itcoincides substantially with the center line of the track to be followedon the information layer 2.

The detection system 8 is now described in further detail: it includesan astigmatism generating element, an astigmatism correcting element 27and a quadrant detector 28.

The astigmatism generating element generates a first amount ofastigmatism W₁ so as to transform the radiation beam 25 to a firstastigmatic radiation beam 29 having a first focal line F₁, and a secondfocal line F₂ which is further from the astigmatism generating element 9than focal line F₁. The distance Δf₁ between the first and second focallines F₁ and F₂ is named hereafter “astigmatic distance.” The firstamount of astigmatism W₁ is represented by a vector (W_(o,1), θ₁) in areference plane XY perpendicular to the optical axis OO′, where W_(o,1)represents the magnitude of W₁ and θ₁ represents the angle between saidfirst focal line and the reference axis X which is perpendicular to theoptical axis OO′.

In the preferred embodiment shown in FIG. 3, the astigmatism generatingelement is formed by the plane parallel plate element 9 which alsooperates as beam splitter (see above). Alternatively, the astigmatismgenerating element may be formed by a cylinder lens, a toroidal element,or a hologram having an anisotropic curvature or pitch of the gratinglines. The magnitude W_(o,1) is expressed in the form of the Seidelcoefficient W₂₂. The following equation gives the root-mean-square valueW_(22rms), normalized with respect to the wavelength λ, of thecoefficient W₂₂:

$\begin{matrix}{W_{22{rms}} = \frac{{NA}^{2}{d\left( {n^{2} - 1} \right)}{\sin^{2}(\alpha)}}{2\lambda\sqrt{24}\left( {n^{2} - {\sin^{2}\alpha}} \right)^{\frac{3}{2}}}} & (3)\end{matrix}$wherein “d” is the thickness of the plane parallel plate, “n” is therefractive index of the plane parallel plate, “α” is the angle of theplane parallel plate with the optical axis (preferably 45°), and “NA” isthe numerical aperture of the radiation beam that is incident to theplane parallel plate. For further information, see e.g. M. Born and E.Wolf, “Principles of Optics,” p.469–470 (6^(th) ed.) (Pergamon Press)(ISBN 0-08-09482-4).

The astigmatism correcting element 27 generates a second amount ofastigmatism W₂ so as to transform the first astigmatic radiation beam 29to a second astigmatic radiation beam 30 having a third amount ofastigmatism W₃. The second amount of astigmatism W₂ is represented by avector (W_(o,2), θ₂) in the reference plane XY, and the third amount ofastigmatism W₃ is represented by a vector (W_(o,3), θ₃) in the referenceplane XY. By definition, the astigmatism correcting element 27 generatesthe amount W₂ so as to transform an incident, non-astigmatic radiationbeam to an emerging astigmatic radiation beam having a third focal lineF3 and a fourth focal line F4 which is further from the element 27 thanthe focal line F3. The distance Δf₂ between the focal lines F3 and F4 isalso referred to as “astigmatic distance.” Similarly, the astigmaticradiation beam 30 has a fifth focal line F5 and a sixth focal line F6;the distance Δf₃ between the focal lines F5 and F6 is also referred toas “astigmatic distance.”

The detector 28 transforms the radiation beam 30 to at least oneelectrical signal. Preferably, the detector 28 is formed by aquadrant-detector having two perpendicular separation lines.

More specifically, in the case where the quadrant detector 28 isarranged for implementing the “astigmatic method,” it includes fourdetector elements C1 through C4 and a first electronic circuit (notshown in FIG. 3 and shown in FIGS. 5–10). The detector elements C1through C4 are arranged in the XY-plane in the form of four separatequadrants as follows: the detector element C1 is diagonally opposed tothe detector element C3; the detector elements C2 is diagonally opposedto the detector elements C4; the separation line between the detectorelements C1 and C4 (as well as that between the detector elements C2 andC3) is parallel to the X-axis; and the separation line between thedetector elements C1 and C2 (as well as that between the detectorelements C3 and C4) is parallel to the Y-axis. The detector elements C1,C2, C3 and C4 are arranged for providing four detector element signalsI_(C1), I_(C2), I_(C3) and I_(C4), respectively, that represent thelight intensity of the radiation beam 14 falling on the pertainingdetector elements. The first electronic circuit is arranged fortransforming the signals I_(C1) through I_(C4) to the focus error signalI_(focus) according to the following equation:I _(focus)=(I _(C1) +I _(C3))−(I _(C2) +I _(C4)).  (4)

The radial-tracking error signal I_(radial) according to the radialpush-pull method can be obtained by combining the signals I_(C1),I_(C2), I_(C3) and I_(C4) as follows:I _(radial)=(I _(C1) +I _(C4))−(I _(C2) +I _(C3)).  (5)

The detection of the signals I_(C1) through I_(C4) depends on the anglesbetween the focal lines F3 and F4 and the separation lines of thedetector elements C1 through C4 (that is, on the angle between the focalline F3 and the X-axis). Notably, one-spot and 3-spot radial trackingmethod can be used as radial push-pull method for forming the signalI_(radial).

Furthermore, W₃ is adapted to the detector 28 so that the angle θ₃equals 45° with the separation lines of the detector 28; the orientationof the separation lines of the detector has been chosen so as to allowthe radial tracking error signal to be generated by the push-pullmethod. According to the invention, W_(o,2) and θ₂ comply substantiallywith the following equation:(W _(o,1), 2θ₁)+(W _(o,2), 2θ₂)=(W _(o,3), 2θ₃)  (6)

FIG. 4 shows the relationship of Equation (6) between the amounts ofastigmatism W₁, W₂ and W₃ in the reference plane XY. The magnitude W₃and the angle θ₃ may be expressed in the plane XY with respect to anyreference direction X chosen arbitrarily: the magnitude W3 and the angleθ₃ are rotation-invariant in the plane XY.

Calculations have been made from Equation (6) where the magnitudeW_(o,1) equals an arbitrarily chosen value, W_(o,1) ^(fix) (=99 μm whenexpressed in longitudinal aberration), the angle θ₁ equals anarbitrarily chosen value, θ₁ ^(fix) (=90°), the magnitude W_(o,3) equala desired value, W_(o,3) ^(des), as required by other parameters in thedevice (1295 μm), and the angle θ₃ equal a desired value, θ₃ ^(des), asrequired by the “astigmatic method” (i.e. 45°). The magnitude W_(o,2)^(cal2) and the angle θ₂ ^(cal2) have been calculated from Equation (6).Table 1 shows these magnitudes and angles.

TABLE 1 W₁ ^(fix) W₂ ^(cal2) W₃ ^(des) W_(o,1) ^(fix) θ₁ ^(fix) W_(o,2)^(cal2) θ₂ ^(cal2) W_(o,3) ^(des) θ₃ ^(des) 99 μm 90° 1322 μm 39° 1295μm 45°

Thus, according to the teaching of the present invention, for an amountof astigmatism W₁ having a magnitude of 99 μm and an angle of 90°, theastigmatism correcting element is to generate an amount of astigmatismhaving a magnitude of 1322 μm and an angle of 39° in order to obtain theamount of astigmatism W₃ as desired, i.e. with a magnitude of 1295 μmand an angle of 45°.

Ray-tracing simulations have been made from the arbitrarily chosenvalues W_(o,1) ^(fix) and θ₁ ^(fix) and the calculated values W_(o,2)^(cal2) and θ₂ ^(cal2): as a result, the value of ma W_(o,3) ^(sim2) andthe value of angle θ₃ ^(sim2) have been obtained. Table 2 shows thesevalues of magnitudes and angles.

TABLE 2 W₁ ^(fix) W₂ ^(cal2) W₃ ^(sim2) W_(o,1) ^(fix) θ₁ ^(fix) W_(o,2)^(cal2) θ₂ ^(cal2) W_(o,3) ^(sim2) θ₃ ^(sim2) 99 μm 90° 1322 μm 39° 1298μm 45°

As shown in Table 2, according to the results of the simulations, if anamount of astigmatism W₁ having a magnitude of 99 μm and an angle of 90°is corrected by an amount of astigmatism W₂ having a magnitude of 1322μm and an angle of 39°, then the resulting amount of astigmatism W₃ hasa magnitude of 1322 μm and an angle of 45°, which substantially equalthe desired values 1295 μm and 45°, respectively. In other words, thereis a difference of 0.2% between the value of the magnitude obtained bycalculations according to the present invention and that obtainedthrough simulations, and there is a difference of less than 1 degreebetween the value of the angle obtained by calculations according to thepresent invention and that obtained through simulations. In the presentdescription, a value of magnitude or angle “substantially equals”another value of magnitude or angle where the difference between the twovalues is less than 5%.

Two embodiments of the astigmatism correcting element according to theinvention are described below. FIG. 5 is a schematic illustration of theastigmatism generating element 9, the first embodiment 27′ of theastigmatism correcting element 27 and the quadrant detector 28. FIG. 6is a schematic illustration of the astigmatism generating element 9, thesecond embodiment 27″ of the astigmatism correcting element 27 and thequadrant detector 28.

As shown in FIG. 5, the astigmatism correcting element 27′ formed by acylindrical surface having an axis of symmetry Δ that forms apredetermined first angle γ_(cyl) with respect of the X-axis in the XYplane. The angle γ_(cyl) is given by the following expression:γ_(cyl)−θ₃=½Arc sin[W _(o,1) sin(2(θ₁−θ₃))/(W _(o,3) ² +W _(o,1) ²−2W_(o,3) W _(o,1) cos(2(θ₁−θ₃)))^(1/2)]where θ₁−θ₃, W_(o,1) and W_(o,3) satisfy the following condition:W _(o,3) ² +W _(o,1) ²>2W _(o,3) W _(o,1) cos(2(θ₁−θ₃))

Furthermore, the cylindrical surface is arranged for generating a thirdastigmatic distance W_(o,2) in order to transform the magnitude W_(o,1)of the astigmatism W₁ to the magnitude W_(o,3) of the astigmatism W₃.More specifically, the astigmatic distance W_(o,2) may be defined asfollows:W _(o,2) =[W _(o,3) ² +W _(o,1) ²−2W _(o,3) W _(o,1)cos(2(θ₁−θ₃))]^(1/2)where θ₁−θ₃, and W_(o,1) and W_(o,3) satisfy the following condition:W _(o,3) ² +W _(o,1) ²>2W _(o,3) W _(o,1) cos(2(θ₁−θ₃)).

Notably, the formulae of γ_(cyl)−θ₃ and W_(o,2) are valid where W_(o,1),W_(o,2) and W_(o,3) are represented by any of the following: a Seidelcoefficient, a Zernike coefficient, a peak-value of the wavefrontaberration, the “astigmatic distance” or longitudinal aberration (whichcorresponds to the distance between the first and second focal lines).

As shown in FIG. 6, the astigmatism correcting element 27″ is formed bya plane surface arranged as follows. The astigmatism generating element9 is formed by a plane parallel plate which is: firstly arranged withthe same orientation as the detector 28 (represented by the dashedsquare), and secondly rotated along the Y-axis so that the normal N1 tothe plate 9 makes an angle α of 45 degrees with respect to the Z-axis.The astigmatism correcting element 27″ is formed by a plane parallelplate which is: firstly arranged with the same orientation as the plate9 (represented by the dashed square), and secondly rotated along theZ-axis so that the normal N2 to the plate 27″ makes an angle β withrespect to the X-axis. Preferably, the angle β equals 45° according tothe astigmatic method.

As a matter of pure illustration, the effect of the correction accordingto the invention is now described in further detail.

FIGS. 7 and 8 are schematic representations of the focal line formed bythe central radiation beam 14 on the quadrant detector 28, with andwithout correction according to the invention, respectively. FIGS. 9 and10 are schematic representations of the two satellite radiation beamsformed on the quadrant detector 28, with and without correctionaccording to the invention, respectively.

FIG. 7 relates to the case where the angle θ₃ between the focal line F3and the separation line between the detector elements C1 and C2, equals45° and the scanning spot 18 is not in focus on the information layer 2.The spot of the astigmatic radiation beam 30 has an elliptic shape S1 onthe detector elements C1 through C4, wherein the main axis of theelliptical shape S1 forms an angle of 45° with respect to the X-axis(this shape corresponding to a focal line of the amount W₃). Therefore,the signal I_(focus) formed according to the Equation (4) differs from0. In other words, detection of the focus error is possible.

FIG. 8 relates to the case where the angle between the focal line F3 andthe separation line between the detector elements C1 and C2 equals 0°and without the astigmatism correction according to the invention. Thus,the spot of the astigmatic radiation beam 30 has an elliptic shape S2 onthe detector elements C1 through C4, wherein the main axis of theelliptical shape S2 forms an angle of 0° with respect to the X-axis.Therefore, the signal I_(focus) formed according to Equation (4) equals0. In other words, detection of the focus error is not possible.

FIG. 9 relates to the case where the angle θ₃ between the focal line F3and the separation line between the detectors C1 and C3 equals 45°.Notably, the direction of the Y-axis (indicated in FIG. 9) correspondsto the tangential direction (that is, the direction tangential to thetrack to be scanned). The first astigmatic radiation beam forms twohalf-lobes S3 and S4 and the second astigmatic radiation beam forms twohalf-lobes S5 and S6. The half-lobes S3 through S6 are oriented so as tobe aligned with the direction of the separation line, that is, with thedirection of the track. Thus, the signal I_(radial) (formed according toEquation (5)) differs from 0. In other words, there is detection of theradial-tracking error signal.

FIG. 10 relates to the case where the angle between the focal line F3and the separation line between the detectors C1 and C2 equals 0° andwithout astigmatism correction according to the invention. Notably, thedirection of the Y-axis (indicated in FIG. 10) corresponds to thetangential direction (that is, the direction tangential to the track tobe scanned). The first astigmatic radiation beam forms two half-lobes S7and S8 and the second astigmatic radiation beam forms two half-lobes S9and S10. The half-lobes S7 through S10 are oriented so as to form anangle of 45° with the direction of the separation line, that is, withthe direction of the track. Thus, the signal I_(radial) (formedaccording to Equation (5)) equals 0. In other words, detection of theradial-tracking error is not possible.

It is to be noted that the shapes shown in FIGS. 7 through 10 correspondto the case where the objective lens 19 is too far from the informationlayer 2. Similar shapes are obtained in the case where the objectivelens 19 is too close to the information layer 2 (symmetry with respectto the Y-axis).

It is to be appreciated that numerous variations and modifications maybe employed in relation to the embodiments described above, withoutdeparting from the scope of the invention which is defined in theappended claims.

As an alternative, the optical scanning device may be of the typecapable to performing simultaneous multi-track scanning. This results inimproving data read-out in the “reading mode” and/or write speed in the“writing mode” as described, for example, in the U.S. Pat. No.4,449,212. The description of the multi-tracking arrangement accordingto the U.S. Pat. No. 4,449,212 is incorporated herein by reference.

As an improvement, the optical scanning device according to theinvention further includes a servo lens having an entrance surfacefacing the astigmatism generating element 9 and an exit surface facingthe quadrant detector 28, wherein the entrance surface of the servo lensis arranged for forming the astigmatism correcting element 27.Alternatively, the exit surface (instead of the entrance surface) may bearranged for forming the astigmatism correcting element 27.

An advantage of using the entrance surface of the servo lens as theastigmatism correcting element 9 is to allow the possibility ofarranging the exit surface of the servo lens for providing an additionaloptical function. For instance, the exit surface may be sphericallycurved.

1. An optical scanning device (1) for scanning an information layer (2),the device including a radiation source (6) for supplying a firstradiation beam (14), a lens system (7) for transforming said firstradiation beam to a scanning spot (18) in the position of saidinformation layer, the lens system having an optical axis (OO′), and adetection system (8) including: an astigmatism generating element (9)for generating a first amount of astigmatism (W₁) so as to transformsaid first radiation beam to a first astigmatic radiation beam (29)having a first focal line (F₁) and a second focal line (F₂) which isfurther from said astigmatism generating element than said first focalline, said first amount of astigmatism (W₁) being represented by avector (W_(o,1), θ₁) in a reference plane (XY) perpendicular to saidoptical axis, where W_(o,i) represents the magnitude of W_(i) and θ_(i)represents the angle between said first focal line and a reference axis(X) which is perpendicular to said optical axis; an astigmatismcorrecting element (27) for generating a second amount of astigmatism(W₂) so as to transform said first astigmatic radiation beam to a secondastigmatic radiation beam (30) having a third amount of astigmatism(W₃), said second amount of astigmatism (W₂) being represented by avector (W_(o,2), θ₂) in said reference plane, and said third amount ofastigmatism (W₃) being represented by a vector (W_(o,3), θ₃) in saidreference plane, and a detector (28) for transforming said thirdradiation beam to an electrical signal, characterized in that W₃ isadapted to said detector and that W_(o,2) and θ₂ comply substantiallywith the following equation:(W _(o,1), 2θ₁)+(W _(o,2), 2θ₂)=(W _(o,3), 2θ₃).
 2. The optical scanningdevice (1) as claimed in claim 1, characterized in that said astigmatismgenerating element (9) includes a first plane parallel plate for use asbeam splitter.
 3. The optical scanning device (1) as claimed in claim 1,characterized in that said astigmatism correcting element (27) is formedby a cylindrical surface having an axis of symmetry that has apredetermined first angle (γ_(cyl)) with to respect said reference axis(X) and in that said predetermined first angle is given by the followingequation:γ_(cyl)−θ₃=½Arc sin[W _(o,1) sin(2(θ₁−θ₃))/(W _(o,3) ² +W _(o,1) ²−2W_(o,3) W _(o,1) cos(2(θ₁−θ₃)))^(1/2)] wherein θ₁−θ₃, W_(o,1) and W_(o,3)satisfy the following condition:W _(o,3) ² +W _(o,1) ²>2W _(o,3) W _(o,1) cos(2(θ₁−θ₃)).
 4. The opticalscanning device (1) as claimed in claim 3, characterized in theastigmatic distance (Δf₂) of said cylindrical surface is given by thefollowing equation:W _(o,2) =[W _(o,3) ² +W _(o,1) ²−2W _(o,3) W _(o,1)cos(2(θ₁−θ₃))]^(1/2) where θ₁−θ₃, W_(o,1), W_(o,3), satisfy thefollowing condition:W _(o,3) ² +W _(o,1) ²>2W _(o,3) W _(o,1) cos(2(θ₁−θ₃)).
 5. The opticalscanning device (1) as claimed in claim 1, characterized in that saidastigmatism correcting element (27) is formed by a second plane parallelplate having a normal direction (N2).
 6. The optical scanning device (1)as claimed in claim 2, characterized in that said astigmatism correctingelement (27) is formed as the entrance surface of a servo lens facingsaid astigmatism generating element (9) and in that said servo lens hasan entrance surface and an exit surface facing said detector (28), oneof these surfaces being anisotropically curved.
 7. The optical scanningdevice (1) as claimed in claim 1, characterized in that said detector(28) is formed by a quadrant detector having a separation line in thedirection of said reference axis (X) and in that W₃ is adapted to saiddetector (28) so that the angle θ₃ substantially equals 45°.
 8. Theoptical scanning device (1) as claimed in claim 1, characterized in thatsaid detection system (8) is further arranged for providing a focuserror signal (I_(focus)) and/or a radial-tracking error signal(I_(radial)) and in that it further includes a servo circuit (10) and anactuator (11, 12) responsive to said focus error signal and/or saidradial-tracking error signal for controlling the position of saidscanning spot (18) with respect to the position of said informationlayer (2) and/or of a track of said information layer which is to bescanned.
 9. The optical scanning device (1) as claimed in claim 1,characterized in that it further includes an information processing unitfor error correction (13).
 10. The optical scanning device (1) asclaimed in claim 1, characterized in that said lens system (7) includesa first objective lens (19) and a second objective lens (21) that form adoublet-lens system.